Black Dwarf Supernovae

14 April, 2021

“Black dwarf supernovae”. They sound quite dramatic! And indeed, they may be the last really exciting events in the Universe.

It’s too early to be sure. There could be plenty of things about astrophysics we don’t understand yet—and intelligent life may throw up surprises even in the very far future. But there’s a nice scenario here:

• M. E. Caplan, Black dwarf supernova in the far future, Monthly Notices of the Royal Astronomical Society 497 (2020), 4357–4362.

First, let me set the stage. What happens in the short run: say, the first 1023 years or so?

For a while, galaxies will keep colliding. These collisions seem to destroy spiral galaxies: they fuse into bigger elliptical galaxies. We can already see this happening here and there—and our own Milky Way may have a near collision with Andromeda in only 3.85 billion years or so, well before the Sun becomes a red giant. If this happens, a bunch of new stars will be born from the shock waves due to colliding interstellar gas.

By 7 billion years we expect that Andromeda and the Milky Way will merge and form a large elliptical galaxy. Unfortunately, elliptical galaxies lack spiral arms, which seem to be a crucial part of the star formation process, so star formation may cease even before the raw materials run out.

Of course, no matter what happens, the birth of new stars must eventually cease, since there’s a limited amount of hydrogen, helium, and other stuff that can undergo fusion.

This means that all the stars will eventually burn out. The longest lived are the red dwarf stars, the smallest stars capable of supporting fusion today, with a mass about 0.08 times that of the Sun. These will run out of hydrogen about 10 trillion years from now, and not be able to burn heavier elements–so then they will slowly cool down.

(I’m deliberately ignoring what intelligent life may do. We can imagine civilizations that develop the ability to control stars, but it’s hard to predict what they’ll do so I’m leaving them out of this story.)

A star becomes a white dwarf—and eventually a black dwarf when it cools—if its core, made of highly compressed matter, has a mass less than 1.4 solar masses. In this case the core can be held up by the ‘electron degeneracy pressure’ caused by the Pauli exclusion principle, which works even at zero temperature. But if the core is heavier than this, it collapses! It becomes a neutron star if it’s between 1.4 and 2 solar masses, and a black hole if it’s more massive.

In about 100 trillion years, all normal star formation processes will have ceased, and the universe will have a population of stars consisting of about 55% white dwarfs, 45% brown dwarfs, and a smaller number of neutron stars and black holes. Star formation will continue at a very slow rate due to collisions between brown and/or white dwarfs.

The black holes will suck up some of the other stars they encounter. This is especially true for the big black holes at the galactic centers, which power radio galaxies if they swallow stars at a sufficiently rapid rate. But most of the stars, as well as interstellar gas and dust, will eventually be hurled into intergalactic space. This happens to a star whenever it accidentally reaches escape velocity through its random encounters with other stars. It’s a slow process, but computer simulations show that about 90% of the mass of the galaxies will eventually ‘boil off’ this way — while the rest becomes a big black hole.

How long will all this take? Well, the white dwarfs will cool to black dwarfs in about 100 quadrillion years, and the galaxies will boil away by about 10 quintillion years. Most planets will have already been knocked off their orbits by then, thanks to random disturbances which gradually take their toll over time. But any that are still orbiting stars will spiral in thanks to gravitational radiation in about 100 quintillion years.

I think the numbers are getting a bit silly. 100 quintillion is 1020, and let’s use scientific notation from now on.

Then what? Well, in about 1023 years the dead stars will actually boil off from the galactic clusters, not just the galaxies, so the clusters will disintegrate. At this point the cosmic background radiation will have cooled to about 10-13 Kelvin, and most things will be at about that temperature unless proton decay or some other such process keeps them warmer.

Okay: so now we have a bunch of isolated black holes, neutron stars, and black dwarfs together with lone planets, asteroids, rocks, dust grains, molecules and atoms of gas, photons and neutrinos, all very close to absolute zero.

I had a dream, which was not all a dream.
The bright sun was extinguishd, and the stars
Did wander darkling in the eternal space,
Rayless, and pathless, and the icy earth
Swung blind and blackening in the moonless air.

— Lord Byron

So what happens next?

We expect that black holes evaporate due to Hawking radiation: a solar-mass one should do so in 1067 years, and a really big one, comparable to the mass of a galaxy, should take about 1099 years. Small objects like planets and asteroids may eventually ‘sublimate’: that is, slowly dissipate by losing atoms due to random processes. I haven’t seen estimates on how long this will take. For larger objects, like neutron stars, this may take a very long time.

But I want to focus on stars lighter than 1.2 solar masses. As I mentioned, these will become white dwarfs held up by their electron degeneracy pressure, and by about 1017 years they will cool down to become very cold black dwarfs. Their cores will crystallize!

Then what? If a proton can decay into other particles, for example a positron and a neutral pion, black dwarfs may slowly shrink away to nothing due to this process, emitting particles as they fade away! Right now we know that the lifetime of the proton to decay via such processes is at least 1032 years. It could be much longer.

But suppose the proton is completely stable. Then what happens? In this scenario, a very slow process of nuclear fusion will slowly turn black dwarfs into iron! It’s called pycnonuclear fusion. The idea is that due to quantum tunneling, nuclei next to each other in the crystal lattice within a black dwarf will occasionally get ‘right on top of each other’ and fuse into heavier nucleus! Since iron-56 is the most stable nucleus, eventually iron will predominate.

Iron is more dense than lighter elements, so as this happens the black dwarf will shrink. It may eventually shrink down to being so dense that electron pressure will no longer hold it up. If this happens, the black dwarf will suddenly collapse, just like heavier stars. It will release a huge amount of energy and explode as gravitational potential energy gets converted into heat. This is a black dwarf supernova.

When will black dwarf supernovae first happen, assuming proton decay or some other unknown processes don’t destroy the black dwarfs first?

This is what Matt Caplan calculated:

We now consider the evolution of a white dwarf toward an iron black dwarf and the circumstances that result in collapse. Going beyond the simple order of magnitude estimates of Dyson (1979), we know pycnonuclear fusion rates are strongly dependent on density so they are greatest in the core of the black dwarf and slowest at the surface. Therefore, the internal structure of a black dwarf evolving toward collapse can be thought of as an astronomically slowly moving ‘burning’ front growing outward from the core toward the surface. This burning front grows outward much more slowly than any hydrodynamical or nuclear timescale, and the star remains at approximately zero temperature for this phase. Furthermore, in contrast to traditional thermonuclear stellar burning, the later reactions with higher Z parents take significantly longer due to the larger tunneling barriers for fusion.

Here “later reactions with higher Z parents” means fusion reactions involving heavier nuclei. The very last step, for example, is when two silicon nuclei fuse to form a nucleus of iron. In an ordinary star these later reactions happen much faster than those involving light nuclei, but for black dwarfs this pattern is reversed—and everything happens at ridiculously slow rate, at a temperature near absolute zero.

He estimates a black dwarf of 1.24 solar masses will collapse and go supernova after about 101600 years, when roughly half its mass has turned to iron.

Lighter ones will take much longer. A black dwarf of 1.16 solar masses could take 1032000 years to go supernova.

These black dwarf supernovae could be the last really energetic events in the Universe.

It’s downright scary to think how far apart these black dwarfs will be when they explode. As I mentioned, galaxies and clusters will have long since have boiled away, so every black dwarf will be completely alone in the depths of space. Distances between them will be doubling every 12 billion years according to the current standard model of cosmology, the ΛCDM model. But 12 billion years is peanuts compared to the time scales I’m talking about now!

So, by the time black dwarfs start to explode, the distances between these stars will be expanded by a factor of roughly

$\displaystyle{ e^{10^{1000}} }$

compared to their distances today. That’s a very rough estimate, but it means that each black dwarf supernova will be living in its own separate world.

The Expansion of the Universe

9 April, 2021

We can wait a while to explore the Universe, but we shouldn’t wait too long. If the Universe continues its accelerating expansion as predicted by the usual model of cosmology, it will eventually expand by a factor of 2 every 12 billion years. So if we wait too long, we can’t ever reach a distant galaxy.

In fact, after 150 billion years, all galaxies outside our Local Group will become completely inaccessible, in principle by any form of transportation not faster than light!

• Toby Ord, The edges of our Universe.

This is where I got the table.

150 billion years sounds like a long time, but the smallest stars powered by fusion—the red dwarf stars, which are very plentiful—are expected to last much longer: about 10 trillion years!  So, we can imagine a technologically advanced civilization that has managed to spread over the Local Group and live near red dwarf stars, which eventually regrets that it has waited too long to expand through more of the Universe.

The Local Group is a collection of roughly 50 nearby galaxies containing about 2 trillion stars, so there’s certainly plenty to do here. It’s held together by gravity, so it won’t get stretched out by the expansion of the Universe—not, at least, until its stars slowly “boil off” due to some randomly picking up high speeds. But will happen much, much later: more than 10 quintillion years, that is, 1019 years.

Great Conjunction

5 December, 2020

I’ve been seeing Saturn and Jupiter coming closer to each other in the sky lately. Jupiter passes by Saturn every 19.6 years, and it’s called a great conjunction. But I just learned that on December 21st they’ll look closer than they have since March 1226! They’ll be just 0.1 degrees apart: 6.1 arcminutes, to be precise. That’s less than a fifth of the Moon’s apparent width.

Here’s the expected view from New York on December 16th, 45 minutes after sunset, when there will also be a crescent Moon:

Jupiter and Saturn were even closer on July 17, 1623—just 5.2 arcminutes apart—but the glare from the the Sun made them invisible from Earth. There will be another close great conjunction on March 15, 2080. Jupiter and Saturn will be just 6.0 arcminutes apart then! If you’re young, maybe you can see that one. Not me.

On February 16, 7541, Jupiter will actually pass in front of part of Saturn! This called a transit. But if you can wait that long, you might as well wait for June 17, 7541, when Jupiter will completely block the view of Saturn. This is called an occultation.

So yes, Jupiter passes by Saturn more than once that year! In fact it’ll do it three times: this is called a triple conjunction. Because the Earth moves around the Sun much faster than Jupiter or Saturn, these planets sometimes seem to move backwards in the sky, and thanks to this, there are some great conjunctions where Jupiter and Saturn come close to each other in the sky three times in rapid succession, like in 1682–1683:

I got this picture from here:

• Patrick Hartigan, Jupiter-Saturn conjunction series from 0 CE to 3000 CE.

You can have a lot of fun reading this. Since Jupiter and Saturn are in a 5:2 orbital resonance—that is, Jupiter orbits the Sun 5 times in the time it takes Saturn to go around twice—the great conjunctions are not random. Instead, they follow interesting patterns!

Puzzle. Why are triple conjunctions more common than double conjunctions?

Diary, 2003-2020

8 August, 2020

I keep putting off organizing my written material, but with coronavirus I’m feeling more mortal than usual, so I’d like get this out into the world now:

• John Baez, Diary, 2003–2020.

Go ahead and grab a copy!

It’s got all my best tweets and Google+ posts, mainly explaining math and physics, but also my travel notes and other things… starting in 2003 with my ruminations on economics and ecology. It’s too big to read all at once, but I think you can dip into it more or less anywhere and pull out something fun.

It goes up to July 2020. It’s 2184 pages long.

I fixed a few problems like missing pictures, but there are probably more. If you let me know about them, I’ll fix them (if it’s easy).

Entropy in the Universe

25 January, 2020

If you click on this picture, you’ll see a zoomable image of the Milky Way with 84 million stars:

But stars contribute only a tiny fraction of the total entropy in the observable Universe. If it’s random information you want, look elsewhere!

First: what’s the ‘observable Universe’, exactly?

The further you look out into the Universe, the further you look back in time. You can’t see through the hot gas from 380,000 years after the Big Bang. That ‘wall of fire’ marks the limits of the observable Universe.

But as the Universe expands, the distant ancient stars and gas we see have moved even farther away, so they’re no longer observable. Thus, the so-called ‘observable Universe’ is really the ‘formerly observable Universe’. Its edge is 46.5 billion light years away now!

This is true even though the Universe is only 13.8 billion years old. A standard challenge in understanding general relativity is to figure out how this is possible, given that nothing can move faster than light.

What’s the total number of stars in the observable Universe? Estimates go up as telescopes improve. Right now people think there are between 100 and 400 billion stars in the Milky Way. They think there are between 170 billion and 2 trillion galaxies in the Universe.

In 2009, Chas Egan and Charles Lineweaver estimated the total entropy of all the stars in the observable Universe at 1081 bits. You should think of these as qubits: it’s the amount of information to describe the quantum state of everything in all these stars.

But the entropy of interstellar and intergalactic gas and dust is about ten times more the entropy of stars! It’s about 1082 bits.

The entropy in all the photons in the Universe is even more! The Universe is full of radiation left over from the Big Bang. The photons in the observable Universe left over from the Big Bang have a total entropy of about 1090 bits. It’s called the ‘cosmic microwave background radiation’.

The neutrinos from the Big Bang also carry about 1090 bits—a bit less than the photons. The gravitons carry much less, about 1088 bits. That’s because they decoupled from other matter and radiation very early, and have been cooling ever since. On the other hand, photons in the cosmic microwave background radiation were formed by annihilating
electron-positron pairs until about 10 seconds after the Big Bang. Thus the graviton radiation is expected to be cooler than the microwave background radiation: about 0.6 kelvin as compared to 2.7 kelvin.

Black holes have immensely more entropy than anything listed so far. Egan and Lineweaver estimate the entropy of stellar-mass black holes in the observable Universe at 1098 bits. This is connected to why black holes are so stable: the Second Law says entropy likes to increase.

But the entropy of black holes grows quadratically with mass! So black holes tend to merge and form bigger black holes — ultimately forming the ‘supermassive’ black holes at the centers of most galaxies. These dominate the entropy of the observable Universe: about 10104 bits.

Hawking predicted that black holes slowly radiate away their mass when they’re in a cold enough environment. But the Universe is much too hot for supermassive black holes to be losing mass now. Instead, they very slowly grow by eating the cosmic microwave background, even when they’re not eating stars, gas and dust.

So, only in the far future will the Universe cool down enough for large black holes to start slowly decaying via Hawking radiation. Entropy will continue to increase… going mainly into photons and gravitons! This process will take a very long time. Assuming nothing is falling into it and no unknown effects intervene, a solar-mass black hole takes about 1067 years to evaporate due to Hawking radiation — while a really big one, comparable to the mass of a galaxy, should take about 1099 years.

If our current most popular ideas on dark energy are correct, the Universe will continue to expand exponentially. Thanks to this, there will be a cosmological event horizon surrounding each observer, which will radiate Hawking radiation at a temperature of roughly 10-30 kelvin.

In this scenario the Universe in the very far future will mainly consist of massless particles produced as Hawking radiation at this temperature: photons and gravitons. The entropy within the exponentially expanding ball of space that is today our ‘observable Universe’ will continue to increase exponentially… but more to the point, the entropy density will approach that of a gas of photons and gravitons in thermal equilibrium at 10-30 kelvin.

Of course, it’s quite likely that some new physics will turn up, between now and then, that changes the story! I hope so: this would be a rather dull ending to the Universe.

For more details, go here:

• Chas A. Egan and Charles H. Lineweaver, A larger estimate of the entropy of the universe, The Astrophysical Journal 710 (2010), 1825.

Also read my page on information.

Ordovician Meteor Event

25 September, 2019

About 1/3 of the meteorites hitting Earth today come from one source: the L chondrite parent body, an asteroid 100–150 kilometers across that was smashed in an impact 468 million years ago. This was biggest asteroid collision in the last 3 billion years!

Here is an L-chondrite:

A chondrite is a stony, non-metallic meteorite that was formed form small grains of dust present in the early Solar System. They are the most common kind of meteorite—and the three most common kinds, each with its own somewhat different chemical composition, seem to come from different asteroids.

L chondrites are named that because they are low in iron. Compared to other chondrites, a lot of L chondrites have been heavily shocked—evidence that their parent body was catastrophically disrupted by a large impact.

It seems that roughly 500,000 years after this event, lots of meteorites started hitting Earth: this is called the Ordovician meteor event. Big craters from that event still dot the Earth! Here are some in North America:

Number 3 is the Rock Elm Disturbance, created when a rock roughly 170 meters in diameter slammed into what’s now Wisconsin:

It doesn’t look like much now, but imagine what it must have been like! The crater is about 6 kilometers across. It features intensely fractured quartz grain and a faulted rim.

It seems these big L-chondrite meteors hit the Earth roughly in a line:

Of course the continents didn’t look like this when the meteor hit, about 467.5 million years ago.

One big question is: was the Ordovician meteor event somehow connected to the giant increase in biodiversity during the Ordovician? Here’s a graph of biodiversity over time:

The Cambrian explosion gets all the press, but in terms of the sheer number of new families the so-called Ordovician radiation was bigger. Most animal life was undersea at the time. This is when coral reefs and other complex ocean ecosystems came into being!

There are lots of theories that try to explain the Ordovician radiation. For example, the oxygen concentration in the atmosphere and ocean soared right before the start of the Ordovician period. More than one of these theories could be right. But it’s interesting to think about the possible influence of the Ordovician meteor event.

There were a lot of meteor impacts, but the dust may have been more important. Right now, extraterrestrial dust counts for just 1% of all dust in the Earth’s atmosphere. In the Ordovician, the amount of extraterrestial dust was 1,000 – 10,000 times greater, due to the big smash-up in the asteroid belt! This may have caused the global cooling we see in that period. The Ordovician started out hot, but by the end there were glaciers.

How could this increase biodiversity? The “intermediate disturbance hypothesis” says that biodiversity increases under conditions of mild stress. Some argue this explains the Ordovician radiation.

I’d say this is pretty iffy. But it’s sure interesting! Read more here:

• Birger Schmitz et al., An extraterrestrial trigger for the mid-Ordovician ice age: Dust from the breakup of the L-chondrite parent body, Science Advances, 18 September 2019.

Another fun question is: where are the remains of the L chondrite parent body? Could they be the asteroids in the Flora family?

Voyager 1

3 September, 2017

Launched 40 years ago, the Voyagers are our longest-lived and most distant spacecraft. Voyager 2 has reached the edge of the heliosphere, the realm where the solar wind and the Sun’s magnetic field live. Voyager 1 has already left the heliosphere and entered interstellar space! A new movie, The Farthest, celebrates the Voyagers’ journey toward the stars:

What has Voyager 1 been doing lately? I’ll skip its amazing exploration of the Solar System….

Leaving the realm of planets

On February 14, 1990, Voyager 1 took the first ever ‘family portrait’ of the Solar System as seen from outside. This includes the famous image of planet Earth known as the Pale Blue Dot:

Soon afterwards, its cameras were deactivated to conserve power and computer resources. The camera software has been removed from the spacecraft, so it would now be hard to get it working again. And here on Earth, the software for reading these images is no longer available!

On February 17, 1998, Voyager 1 reached a distance of 69 AU from the Sun — 69 times farther from the Sun than we are. At that moment it overtook Pioneer 10 as the most distant spacecraft from Earth! Traveling at about 17 kilometers per second, it was moving away from the Sun faster than any other spacecraft. It still is.

That’s 520 million kilometers per year — hard to comprehend. I find it easier to think about this way: it’s 3.6 AU per year. That’s really fast… but not if you’re trying to reach other stars. It will take 20,000 years just to go one light-year.

Termination shock

As Voyager 1 headed for interstellar space, its instruments continued to study the Solar System. Scientists at the Johns Hopkins University said that Voyager 1 entered the termination shock in February 2003. This is a bit like a ‘sonic boom’, but in reverse: it’s the place where the solar wind drops to below the speed of sound. Yes, sound can move through the solar wind, but only sound with extremely long wavelengths — nothing you can hear.

Some other scientists expressed doubt about this, and the issue wasn’t resolved until other data became available, since Voyager 1’s solar-wind detector had stopped working in 1990. This failure meant that termination shock detection had to be inferred from the other instruments on board. We now think that Voyager 1 reached the termination shock on December 15, 2004 — at a distance of 94 AU from the Sun.

Heliosheath

In May 2005, a NASA press release said that Voyager 1 had reached the
heliosheath
. This is a bubble of stagnant solar wind, moving below the speed of sound. It’s outside the termination shock but inside the heliopause, where the interstelllar wind crashes against the solar wind.

On March 31, 2006, amateur radio operators in Germany tracked and received radio waves from Voyager 1 using a 20-meter dish. They
checked their data against data from the Deep Space Network station in Madrid, Spain and yes — it matched. This was the first amateur tracking of Voyager 1!

On December 13, 2010, the the Low Energy Charged Particle device
aboard Voyager 1 showed that it passed the point where the solar wind flows away from the Sun. At this point the solar wind seems to turn sideways, due to the push of the interstellar wind. On this date, the spacecraft was approximately 17.3 billion kilometers from the Sun, or 116 AU.

In March 2011, Voyager 1 was commanded to change its orientation to measure the sideways motion of the solar wind. How? I don’t know. Its solar wind detector was broken.

But anyway, a test roll done in February had confirmed the spacecraft’s ability to maneuver and reorient itself. So, in March it rotated 70 degrees counterclockwise with respect to Earth to detect the solar wind. This was the first time the spacecraft had done any major maneuvering since the family portrait photograph of the planets was taken in 1990.

After the first roll the spacecraft had no problem in reorienting itself with Alpha Centauri, Voyager 1’s guide star, and it resumed sending transmissions back to Earth.

On December 1, 2011, it was announced that Voyager 1 had detected the first Lyman-alpha radiation originating from the Milky Way galaxy. Lyman-alpha radiation had previously been detected from other galaxies, but because of interference from the Sun, the radiation from the Milky Way was not detectable.

Puzzle: What the heck is Lyman-alpha radiation?

On December 5, 2011, Voyager 1 saw that the Solar System’s magnetic field had doubled in strength, basically because it was getting compressed by the pressure of the interstellar wind. Energetic particles originating in the Solar System declined by nearly half, while the detection of high-energy electrons from outside increased 100-fold.

Heliopause and beyond

In June 2012, NASA announced that the probe was detecting even more charged particles from interstellar space. This meant that it was getting close to the heliopause: the place where the gas of interstellar space crashes into the solar wind.

Voyager 1 actually crossed the heliopause in August 2012, although it took another year to confirm this. It was 121 AU from the Sun.

What’s next?

In about 300 years Voyager 1 will reach the Oort cloud, the region of frozen comets. It will take 30,000 years to pass through the Oort cloud. Though it is not heading towards any particular star, in about 40,000 years it will pass within 1.6 light-years of the star Gliese 445.

NASA says:

The Voyagers are destined — perhaps eternally —
to wander the Milky Way.

That’s an exaggeration. The Milky Way will not last forever. In just 3.85 billion years, before our Sun becomes a red giant, the Andromeda galaxy will collide with the Milky Way. In just 100 trillion years, all the stars in the Milky Way will burn out. And in just 10 quintillion years, the Milky Way will have disintegrated, with all the dead stars either falling into black holes or being flung off into intergalactic space.

But still: the Voyagers’ journeys are just beginning. Let’s wish them a happy 40th birthday!

My story here is adapted from this Wikipedia article:

• Wikipedia, Voyager 1.

• NASA, NASA and iconic museum honor Voyager spacecraft 40th anniversary, August 30, 2017.

14 January, 2017

This blog post is based on a thread in the Azimuth Forum.

The current theories about the Sun’s life-time indicate that the Sun will turn into a red giant in about 5 billion years. How and when this process is going to be destructive to the Earth is still debated. Apparently, according to more or less current theories, there has been a quasilinear increase in luminosity. On page 3 of

• K.-P. Schröder and Robert Connon Smith, Distant future of the Sun and Earth revisited, 2008.

The present Sun is increasing its average luminosity at a rate of 1% in every 110 million years, or 10% over the next billion years.

In the Azimuth Forum I asked for information about solar irradiance measurements . Why I was originally interested in how bright the Sun is shining is a longer story, which includes discussions about the global warming potential of methane. For this post I prefer to omit this lengthy historical survey about my original motivations (maybe I’ll come back to this later). Meanwhile there is an also a newer reason why I am interested in solar irradiance measurements, which I want to talk about here.

Strictly speaking I was not only interested in knowing more about how bright the sun is shining, but how bright each of its ‘components’ is shining. That is, I wanted to see spectrally resolved solar irradiance measurements—and in particular, measurements in the range between the wavelengths of roughly 650 and 950 nanometers.

This led me to the the Sorce mission, which is a NASA sponsored satellite mission, whose website is located at the University of Colorado. The website very nicely provides an interactive interface including a fairly clear and intuitive LISIRD interactive app with which the spectral measurements of the Sun can be studied.

As a side remark I should mention that this NASA mission belongs to the NASA Earth Science mission, which is currently threatened to be scrapped.

By using this app, I found in the 650–950 nanometer range a very strange rise in radiation between 2003 and 2016, which happened mainly in the last 2-3 years. You can see this rise here (click to enlarge):

spectral line 774.5nm from day 132 to 5073, day 132 starting Jan 24 in 2003, day 5073 is end of 2016

Now, fluctuations within certain spectral ranges within the Sun’s spectrum are not news. Here, however, it looked as if a rather stable range suddenly started to change rather “dramatically”.

I put the word “dramatically” in quotes for a couple of reasons.

Spectral measurements are complicated and prone to measurement errors. Subtle issues of dirty lenses and the like are already enough to suggest that this is no easy feat, so that this strange rise might easily be due to a measurement failure. Moreover, as I said, it looked as this was a fairly stable range over the course of ten years. But maybe this new rise in irradiation is part of the 11 years solar cycle, i.e., a common phenomenon. In addition, although the rise looks big, it may overall still be rather subtle.

So: how subtle or non-subtle is it then?

In order to assess that, I made a quick estimate (see the Forum discussion) and found that if all the additional radiation would reach the ground (which of course it doesn’t due to absorption), then on 1000 square meters you could easily power a lawn mower with that subtle change! I.e., my estimate was 1200 watts for that patch of lawn. Whoa!

That was disconcerting enough to download the data and linearly interpolate it and calculate the power of that change. I wrote a program in Javascript to do that. The computer calculations revealed an answer of 1000 watts, i.e., my estimate was fairly close. Whoa again!

How does this translate to overall changes in solar irradiance? Some increase had already been noticed. NASA wrote 2003 on its webpage:

Although the inferred increase of solar irradiance in 24 years, about 0.1 percent, is not enough to cause notable climate change, the trend would be important if maintained for a century or more.

That was 13 years ago.

I now used my program to calculate the irradiance for one day in 2016 between the wavelengths of 180.5 nm and 1797.62 nm, a quite big part of the solar spectrum, and got the value 627 W/m2. I computed the difference between this and one day in 2003, approximately one solar cycle earlier. I got 0.61 W/m2, which is 0.1% in 13 years, rather then 24 years. Of course this is not an average value, and not really well adjusted to the sun cycle, and fluctuations play a big role in some parts of the spectrum, but well—this might indicate that the overall rate of rise in solar radiation may have doubled. Likewise concerning the question of the sun’s luminosity: for assessing luminosity one would need to take the concrete satellite-earth orbit at the day of measurement into account, as the distance to the sun varies. But still, on a first glance this all appears disconcerting.

Given that this spectral range has for example an overlap with the absorption of water (clouds!), this should at least be discussed.

See how the spectrum splits into a purple and dark red line in the lower circle? (Click to enlarge.)

Difference in spectrum between day 132 and 5073

The upper circle displays another rise, which is discussed in the forum.

So concluding, all this looks as if this needs to be monitored a bit more closely. It is important to see whether these rises in irradiance are also displayed in other measurements, so I asked in the Azimuth Forum, but so far have gotten no answer.

In short: if you know about publicly available solar spectral irradiance measurements other than the LISIRD ones, then please let me know.

Shock Breakout

30 March, 2016

Here you can see the brilliant flash of a supernova as its core blasts through its surface. This is an animated cartoon made by NASA based on observations of a red supergiant star that exploded in 2011. It has been sped up by a factor of 240. You can see a graph of brightness showing the actual timescale at lower right.

When a star like this runs out of fuel for nuclear fusion, its core cools. That makes the pressure drop—so the core collapses under the force of gravity.

When the core of a supernova collapses, the infalling matter can reach almost a quarter the speed of light. So when it hits the center, this matter becomes very hot! Indeed, the temperature can reach 100 billion kelvin. That’s 6000 times the temperature of our Sun’s core!

For a supernova less than 25 solar masses, the collapse stops only when the core is compressed into a neutron star. As this happens, lots of electrons and protons become neutrons and neutrinos. Most of the resulting energy is instantly carried away by a ten-second burst of neutrinos. This burst can have an energy of 1046 joules.

It’s hard to comprehend this. It’s what you’d get if you suddenly converted the mass of 18,000 Earths into energy! Astronomers use a specially huge unit with such energies: the foe, which stands for ten to the fifty-one ergs.

That’s 1044 joules. So, a supernova can release 100 foe in neutrinos. By comparison, only 1 or 2 foe come out as light.

Why? Neutrinos can effortlessly breeze through matter. Light cannot! So it takes longer to actually see things happen at the star’s surface—especially since a red supergiant is large. This one was about 500 times the radius of our Sun.

So what happened? A shock wave rushed upward through the star. First it broke through the star’s surface in the form of finger-like plasma jets, which you can see in the animation.

20 minutes later, the full fury of the shock wave reached the surface—and the doomed star exploded in a blinding flash! This is called the shock breakout.

Then the star expanded as a blue-hot ball of plasma.

Here’s how the star’s luminosity changed with time, measured in multiples of the Sun’s luminosity:

Note that while the shock breakout seems very bright, it’s ultimately dwarfed by the luminosity of the expanding ball of plasma. So, KSN2011d was actually one of the first two supernovae for which the shock breakout was seen! For details, read this:

• P. M. Garnavich, B. E. Tucker, A. Rest, E. J. Shaya, R. P. Olling, D. Kasen and A. Villar, Shock breakout and early light curves of Type II-P supernovae observed with Kepler.

A Type II supernova is one that shows hydrogen in its spectral lines: these are commonly formed by the collapse of a star that has run out of fuel in its core, but retains hydrogen in its outer layers. A Type II-P is one that shows a plateau in its light curve: the P is for ‘plateau’. These are more common than the Type II-L, which show a more rapid (‘linear’) decay in their luminosity:

Hard X-Ray Burst

25 February, 2016

I just learned something cool: 0.4 seconds after LIGO saw those gravitational waves on 14 September 2015, a satellite named Fermi detected a burst of X-rays!

• V. Connaughton et al, Fermi GBM observations of LIGO gravitational wave event GW150914.

It lasted one second. It was rather weak (for such things). The photons emitted ranged from 50 keV to 10 MeV in energy, with a peak around 3.5 MeV. The paper calls this event a ‘hard X-ray source’. Wikipedia says photons with an energy over 100 keV deserve the name gamma rays, while those between 10 keV and 100 keV are ‘hard X-rays’. So, maybe this event deserves to be maybe a gamma ray burst. I suppose it’s all just a matter of semantics: it’s not as if there’s any sharp difference between a highly energetic X-ray and a low-energy gamma ray.

Whatever you call it, this event does not appear connected with other previously known objects. It’s hard to tell exactly where it happened. But its location is consistent with what little we know about the source of the gravitational waves.

If this X-ray burst was caused by the same event that created the gravitational waves, that would be surprising. Everyone assumed the gravitational waves were formed by two large black holes that had been orbiting each other for millions or billions of years, slowly spiraling down. In this scenario we don’t expect much electromagnetic radiation when the black holes finally collide.

Perhaps those expectations are wrong. Or maybe—just maybe—both the gravitational waves and X-rays were formed during the collapse of a single very large star! That’s what typically causes gamma ray bursts—we think. But it’s not at all typical—as far as we know—for a large star to form two black holes when it collapses! And that’s what we’d need to get that gravitational wave event: two black holes, which then spiral down and merge into one!

Here’s an analysis of the issue:

As he notes, the collapsing star would need to have an insane amount of angular momentum to collapse into a dumb-bell shape and form two black holes, each roughly 30 times the mass of our Sun, which then quickly spiral down and collide.

Furthermore, as Tony Wells pointed to me, the lack of neutrinos argues against the idea that this event involved a large collapsing star:

• ANTARES collaboration, High-energy neutrino follow-up search of Gravitational wave event GW150914 with ANTARES and IceCube.

To add to the muddle, another satellite devoted to observing gamma rays, called INTEGRAL, did not see anything:

It will take a while to sort this out.

But luckily, the first gravitational wave burst seen by LIGO was not the only one! Dennis Overbye of the New York Times writes:

Shortly after the September event, LIGO recorded another, weaker signal that was probably also from black holes, the team said. According to Dr. Weiss, there were at least four detections during the first LIGO observing run, which ended in January. The second run will begin this summer. In the fall, another detector, Advanced Virgo, operated by the European Gravitational Observatory in Italy, will start up. There are hopes for more in the future, in India and Japan.

So we will know more soon!

For more on Fermi: